The role of ethylene glycol in society has advanced considerably from the time when shortly after its first synthesis the molecule's properties became the subject of a fierce dispute between two giants of early organic chemistry, Adolphe Wurtz and Hermann Kolbe. In particular, Wurtz and Kolbe disputed ethylene glycol's functionality and chemical formula among the still emerging understanding of alcohol homologs, which were key to Kolbe's greater theories about chemical structure. The laboratory soon became a proxy war of the rising industrial and technical might of the growing rivalry between Germany and France with both countries devoting considerable resources to the scientific investigation and Wurtz's wizardry at chemical synthesis giving France a considerable advantage. The dispute was ended only by resort to arms when Bismarck's German Confederation annexed Wurtz's Alsatian homeland as a result of the Franco-Prussian war and thus, essentially turned an international dispute into a domestic one.
Today, interest in ethylene glycol is more peaceful but all the more competitive because ethylene glycol is one of the most widely produced organic chemicals. Since large scale industrial production of ethylene glycol began on the eve of the First World War, dramatic increases in the use of the internal combustion engine to power automobiles and other vehicles has spurred demand for ethylene glycol as a coolant and antifreeze. Since then, the increase in the production of ethylene glycol has only accelerated, so that by 2017, the estimated worldwide production of ethylene glycol was in excess of 25 billion tons.
Ethylene glycol is typically prepared as one of many of the derivatives of ethylene oxide, and though other production routes are available, most is produced from ethylene oxide in a liquid phase non-catalytic thermal hydration process. Because ethylene oxide reacts with ethylene glycols more readily than it reacts with water it is inevitable that a mixture of monoethylene glycol as well as higher glycol coproducts, such as diethylene glycol, triethylene glycol and yet still higher ethylene glycols will be formed. Although these higher glycols have considerable economic value, many producers and plant operators wish to avoid producing them because the end-user market for these products is not as well developed and it may be difficult to find and distribute these higher glycols to industrial consumers who have a need for them.
In order to suppress the reaction between product glycol and ethylene oxide and thereby reduce the formation of these higher glycols, conventional non-catalytic hydration are performed with an amount of water that far exceeds the stoichiometric amount of water for the hydration of ethylene oxide to ethylene glycol, e.g., 15 to 40 moles of water per mole of ethylene oxide. This addition of excess water is effective at balancing the kinetically-favored competing reaction between product glycol and ethylene oxide, which as mentioned above competes with the hydration of ethylene oxide to monoethylene glycol. However, while effective at suppressing the production of higher ethylene glycols, using a large excess of water relative to ethylene oxide presents a problem for the plant operator in removing these large excesses of unreacted water because such removal is energy intensive and requires large-scale evaporation/distillation facilities. Accordingly, there has been interest in alternatives to thermal hydration of ethylene oxide for the production of ethylene glycol, such as the homogeneous catalytic hydration of ethylene oxide to monoethylene glycol.
The earliest examples of this approach included the homogeneous catalysis of sulfuric acid and their associated salts (see Othmer, D. F. and Thakar, M. S., Glycol Production—Hydration of Ethylene Oxide. Ind. Eng. Chem. 1958, 50, 1235) European Patent No. 0 123 700 described refinements of these earlier generations of acid catalysts by treating them with, e.g., ethylamines to partially neutralize them in the hope of improving the selectivity of the hydration reaction to monoethylene glycol. Since then other salts have been proposed for homogeneous systems, such as quaternary phosphonium salts as described in U.S. Pat. No. 4,160,116 and metallate and bicarbonate salts as described in U.S. Pat. No. 7,683,221. Increasingly creative combinations of organic species such as EDTA and Salen compounds have also been proposed as homogeneous catalysts (see Hal, J. W., Ledford, J. S., and Zhang, X., Catalysis Today 123 (2007), 310-315).
Homogeneous catalyst systems are often utilized in a two-step process for manufacturing ethylene glycol, see e.g., U.S. Pat. No. 4,519,875 in which ethylene oxide is first reacted with carbon dioxide to manufacture ethylene carbonate, which is then hydrolyzed to ethylene glycol, with typically the same catalyst being used in both steps. Following this pioneering patent, continued research has produced incremental refinements in the two-step process. For example, in U.S. Pat. No. 5,763,691, the carbonation reaction is catalyzed in the ethylene oxide absorbate in the presence of a halogenated organic phosphonium salt carbonation catalyst. Additional research has considerably expanded the scope of known catalysts; see for example macrocyclic chelating compounds (“crown ethers”) described in U.S. Pat. No. 7,453,015.
While homogeneous catalysts improved the selectivity of towards monoethylene glycol compared to non-catalytic therm hydration, the homogeneous catalyst hydration processes have the drawback of adding considerably more process complexity. In addition to the multi-step, multi-reaction catalytic hydration steps mentioned above, there is yet an additional multiplicity of steps after the reaction has been completed. As just one example: the glycol product solution produced by the reaction contains soluble or suspended homogeneous catalyst. This necessitates an additional step of separating the homogeneous catalyst from the glycol product solution which increases the cost and complexity of the process. The additional complexity of the homogeneous catalyst hydration brings with it the additional flaw that it is neither very versatile nor flexible. In particular the two-step hydration process also lacks the versatility to be used both for new plants and revamps. In order to attain the full benefits of using this two-step process to revamp an existing ethylene glycol plant the entire reaction and evaporation section of the existing plant would have to be removed and replaced by new process sections. Thus, this two-step hydration process can be used only for new, “grass roots” plants and cannot be used for revamps because in the case of revamps it would be necessary to remove and replace the entire “back-end,” making such a revamp cost-prohibitive.
Thus, for an existing plant operator seeking to supplement or replace the existing thermal, non-catalytic hydration process with a catalytic process this two-step hydration process will not be suitable. There exists a need in the art for a way for a plant operator to improve the selectivity to monoethylene glycol of an existing non-catalytic, thermal process.
An improved process has been discovered in the present invention for the efficient and selective hydration of ethylene oxide to ethylene glycol. This improved process allows for existing plants using conventional non-catalytic hydration processes to be revamped to incorporate a catalytic hydration process that either replaces or supplements the non-catalytic process with a heterogeneous catalytic hydration process. These heterogeneous catalytic process are considerably easier to operate because they do not require multiple catalysis steps, homogeneous catalyst separation steps and other process steps.